1. Field of the Invention
The present invention is directed to an improvement in the fabrication of metallic and advanced composite components, and more particularly, but not by way of limitation to a metallic bond tool having an improved protective release coating for adhesively bonding metallic and advanced composite components on a bonding surface and its method of manufacture.
2. Description of the Related Art
It is known in the art of manufacturing adhesively bonded metallic and advanced composite components, such as those commonly used in aerostructures, to provide a metallic bond tool having at least one metallic bonding surface for receiving elements of the metallic and advanced composite component to be bonded together by being subjected to elevated temperatures and pressures. Such a metallic bond tool receives a protective release coating on such bonding surface as well as on any complementary item such as a throw in block or other detail that is positioned on such bonding surface to cooperate therewith in the positioning and adhesively bonding together of the metallic and advanced composite component.
At times such a metallic or advanced composite component will comprise a layer or layers of a carbon fiber material that has been pre-impregnated with an uncured resin material, such material being commonly known as xe2x80x9cprepregxe2x80x9d. Such initial layer or layers of prepreg are then followed by the layup thereon of other elements of the component such as a layer of metallic honeycomb core, detail parts, doublers and an overlay of plies of prepreg. The component is thus assembled on the bonding surface of the metallic bond tool and is subsequently covered by a flexible sheet or bag material which is sealed at its edge to the bonding surface of the bond tool with a suitable sealing material such as tape. The component is then considered to be xe2x80x9cbaggedxe2x80x9d. The bag is provided with various tubes which permit the pressure within such bag to be controlled and to permit the expulsion of volatiles when the bagged component secured to the bonding surface of the bond tool is positioned within an autoclave and subjected to an elevated temperature and elevated pressure in a predetermined timed cycle to cure a bonded component. A typical temperature would be 350xc2x0 F. at 35 to 70 psi for a predetermined timed cycle. After the part has been cured the bond tool is removed from the oven and the bag material and tape are removed to permit the component to be removed from the bond tool for further processing.
Another typical metallic and advanced composite component that is manufactured in this manner is the example of positioning a metallic honeycomb core portion directly upon the bonding surface of the bond tool and then laying up layers of prepreg over such honeycomb core in a preferred orientation and then after performing the desired bagging this part is cured in the manner previously described. After debagging such part it is used in connection with the fabrication of a subsequent component.
Quite obviously it is desired to preclude to the greatest extent possible any tendency of the cured component to bond directly in any extent to the bonding surface and to also enhance the removal of the bagging tape and any residue remaining from the bagging and adhesive bonding operation. To accomplish these desired objectives, it is known to apply a protective release agent directly to the bonding surface. One example of such a protective release agent that is commonly used on such bonding tools is hand applied and air dried. However, such a material exudes environmentally undesirable emissions such as dibutylether during application and d-limone during cleaning. Further, such a release material has shown a tendency to transfer to bonding edges of the metallic honeycomb core in applications such as noted above and are difficult or impossible to easily remove thereby requiring the bonded part to be scrapped.
To overcome this problem it has been found necessary to provide as a release agent a non stretchable film containing a fiberglass scrim material. While this material precludes the transfer of the release coating material to the edges of the metallic honeycomb core portion, it is been found that such material is relatively easily damaged by the sharp edges of the honeycomb core and must be periodically replaced. To replace such material requires first the removal of such material in a operation that typically requires in excess of 8 worker hours and the subsequent replacement in a an operation that also requires 8-12 worker hours for a typical tool.
Other disadvantages of the use of such materials involves the cost and difficulty of the disposal of such hazardous wastes. The removal of such release materials from the metallic bonding surface involves laborious scraping and sanding that result in employee exposure to undesired particulates.
Thus, it is evident that a need exists for an improved release coating for the bonding tools used in the manufacture of advanced composite components, as for the aerospace industry, which overcomes the numerous problems associated with the known release coatings and which in addition to providing a release agent that has better release properties also is significantly more durable and thereby eliminating many of the environmental and safety issues presented by the release coatings presently employed.
It is therefore an objective of the present invention to provide a new and improved metallic bond tool having an improved protective release coating for adhesively bonding metallic and advanced composite components on a bonding surface and its method of manufacture.
The foregoing problems are overcome and other advantages are provided by a new improved metallic bond tool having an improved ceramic release coating.
Briefly stated, the present invention contemplates a process for making a metallic bond tool having a protective release coating for adhesively bonding metallic and advanced composite components in an autoclave at an elevated temperature and elevated pressure. The process includes the initial step of cleaning a metal bond tool having at least one metallic bonding surface for receiving elements of a metallic or advanced composite to be adhesively bonded together. This cleaning step is performed by heating the metallic tool to an elevated temperature to burn off any foreign residue or machining oil that might be present on the bond tool. The bond surface is then grit blasted with a suitable grit and blown dry with clean dry air. The bond surface is then lightly sanded as necessary to remove any remaining imperfections and blown clean.
The bond surface then receives a sprayed coat of a ceramic reinforced tetrapolyfluoroethylene primer solution to a desired uniform thickness. The primer coat is then air dried for a predetermined time and thereafter force dried in an oven at an elevated temperature that is below any temperature that would materially effect the metal characteristics such as by annealing such bond tool. The primer coat is then followed by application of a polytetrafluoroethylene containing mid coat solution that is sprayed on the bond tool surface in two box coats. The mid coat application is then followed by the application of a top coat of a polytetrafluoroethylene containing solution while the mid coat is still wet. The coated bond tool is air dried for a predetermined period of time and then cured in an oven at an elevated temperature that is sufficient to cure the mid and top coats and yet is below any temperature that might materially effect adversely the metal characteristics of the metallic bond tool.
The coated metallic bond tool is then buffed with a heavy organic cloth material and is then rinsed with water to remove any polytetrafluoroethylene dust generated by such buffing. Any bonding aids that would normally be secured to the bonding surface to assist in locating and positioning elements of the advanced composite component to be adhesively bonded together, such as throw in blocks, are coated in the same manner. Thus, the present invention provides a metallic bond tool that is provides an improved bonding surface that has excellent durability, does not transfer to the component being bonded together and easily releases elements such as tapes used to bag the advanced composite component for curing without leaving undue residue on the tool. Any residue left on the tool can be cleaned off within a few minutes thereby enabling the improved bond tool to be immediately recycled into production.
The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood so that the present contribution to the art may be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the disclosed specific embodiments may be readily utilized as a basis for modifying or designing other structures and methods for carrying out the same purposes of the present invention. It should be also be realized by those skilled in the art that such equivalent constructions and methods do not depart from the spirit and scope of the invention as set forth in the appended claims.